Technology has been known in which a NOx purification catalyst called a three-way catalyst (hereinafter referred to as TWC), NOx occlusion/reduction catalyst (hereinafter referred to as LNT) or NOx adsorption/reduction catalyst (hereinafter referred to as LNC) is installed in the exhaust path of an engine, and switching control of the exhaust A/F to rich or lean is intermittently performed. The NOx purification catalyst purifies NOx in the exhaust gas by trapping the NOx in the exhaust gas when lean and reducing the NOx thus trapped when rich.
Methods of controlling the exhaust A/F to rich that can be exemplified are a method of controlling the exhaust A/F by adjusting the fuel injection (hereinafter referred to as main injection) amount contributing to torque (hereinafter referred to s rich combustion) after causing the intake air amount to decrease by closing the throttle valve or the like, and a method of causing uncombusted fuel to flow into the exhaust path to control the exhaust A/F to rich, by injecting additional fuel not contributing to torque (hereinafter referred to as post injection) into the cylinders by way of fuel injectors close to when each cylinder of the engine is transitioning to the exhaust stroke from the power stroke (hereinafter referred to as post rich).
In addition, technology has been known whereby a portion of the exhaust gas of the engine is returned to the intake, and the combustion temperature in the cylinders is made to decrease by mixing new air and exhaust gas, whereby the NOx emitted from the engine is decreased (hereinafter referred to as EGR). According to this EGR technology, the exhaust A/F can be controlled to rich by causing the intake air amount to decrease by increasing the exhaust gas amount being recirculated (hereinafter referred to as EGR amount), whereby NOx emissions can be decreased.
Studies with the objective of reducing NOx emissions further have progressed by combining control technology for exhaust A/F using the above-mentioned NOx purification catalyst and EGR technology. For example, an exhaust emission control device has been proposed that includes an EGR device executing switching between recirculation of exhaust gas from an upstream side of a NOx purification catalyst and recirculation of exhaust gas from a downstream side of the NOx purification catalyst (refer to Patent Document 1). With this exhaust emission control device, control is executed such that the exhaust gas from the upstream side of the NOx purification catalyst is made to recirculate during low engine load, and the exhaust gas from the downstream side of the NOx purification catalyst is made to recirculate during high engine load.
In addition, studies have progressed into applying control technology for exhaust A/F using the above-mentioned NOx purification catalyst and EGR technology to an engine that implements forced induction by way of a turbocharger. For example, in engines implementing forced induction by way of a turbocharger, both high-pressure EGR (hereinafter referred to as HP-EGR) introduction in which a portion of the exhaust gas is taken from upstream of the turbine and returned to the intake path, and low-pressure EGR (hereinafter referred to as LP-EGR) in which a portion of the exhaust gas is taken from downstream of the turbine and returned to the intake path are known (refer to Patent Documents 2 to 4). Switching between an HP-EGR path and LP-EGR path is executed according to operating states such as of the engine revolution speed and engine load.    Patent Document 1: Japanese Patent No. 2675405    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-140703    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-150319    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2005-127247